The Havelange deep well and the 1978 Famenne seismic data (Condroz, Belgium)

The Famenne seismic survey (1978) has identified a major subhorizontal reflector, marking an angular unconformity, which was thought to represent the Condroz Overthrust. The outcrop of this thrust fault to the north marks the front of the Variscan Orogenic belt. The aim of the Havelange well was to reach the thrust fault and to determine whether the rock mass underneath was of Caledonian or Hercynian age, the latter possibility being not without economic interest. The well starts within the Upper Devonian and then reaches Lower Devonian formations down to the total depth of 5648 m measured depth (MD). Palynological data have demonstrated the existence of three main faults at 4850 m, 4955 m and 5078 m MD. The positions of these faults have been confirmed by different geophysical well measurements. Sonic logging and well seismic including Vertical Seismic Profiling (VSP) and Offset Vertical Seismic Profiling (OVSP) permit a good comparison between the well data and the Famenne surface seismics. In particular, several Devonian lithofacies can be identified on the seismic section. Near the well location the major subhorizontal thrust has developed into a stack of several thrust slices, the base of which corresponds to the top of hard metamorphic rock, reached in the well at 5450 m MD.